The present invention relates to a system for detecting changes in mechanical impedance and, more particularly, for tracing uterine contractions.
Uterine contraction, although a gross mechanical phenomenon, has proved awkward to measure. Typically, uterine contractions are traced by measuring the spring resistance to a tocotransducer probe which pushes against the maternal abdomen, as disclosed in Hewlett-Packard, "Cardiotocograph", Application Note 700 F., 1979. The probe is typically held in place and against the maternal abdomen by an elastic belt. When contractions occur, the probe encounters a more resistive medium and it moves orthogonal to the abdominal surface. This method generally suffices to trace uterine contraction. However, it is awkward to position the elastic belt and probe properly. Furthermore, the pressure of the elastic belt and probe are a source of discomfort to the subject.
Another widely employed method of tracing contractions is even more intrusive. Intrauterine measurement of contractions can be performed using a balloon-tipped or open-ended fluid filled catheter, as disclosed by D. O. Thorne, I. Assadi, J. Flores, and J. Seitchik, "The relationship of the maximum amplitude and the maximum and minimum slope of the intrauterine pressure waveform in late pregnancy and labor", IEEE Transactions on Biomedical Engineering., vol. BME-19, p. 388, 1972.
More recently, changes in the electrical activity of tissue during contractions have been employed in tracing contractions, as disclosed by C. Marque, J. M. G. Duchene, S. Leclercq, G. S. Panczer, and J. Chaumont, in "Uterine EHG Processing for Obstetrical Monitoring", IEEE Transactions on Biomedical Engineering, Vol. BME-33, No. 12, December 1986. The electrical activity is recorded as a electromyogram, also referred to as a "electrohysterogram" or "EHG".
EHG measurements can be made using intrauterine or abdominal electrodes. Electrical signals at several frequencies have been observed to correlate with contractions. The main problems with EHG measurements are that the signals are not strong, so that they are easily interfered with by electrical signals from spurious physiological activity, and that their correlation with contractions is not very strong. Furthermore, intrauterine electrodes are more intrusive than desired. Abdominal electrodes are less intrusive, but pick up electrical activity resulting from other sources, such as skin stretching, respiratory movements, and movement of abdominal muscles. Because of the weakness of the electrical signals being monitored and the susceptibility of the signals to noise from sources other than the contractions of interest, the sensitivity and validity of abdominal EHG measurements are limited.
What is needed is an improved system and method for tracing uterine contractions which provides accurate tracing and which is also non-intrusive, comfortable and easy to use.